Verifying in-app purchase from Google Play

Question

I'm working on an Azure Functions app that will make calls to Google to verify in-app purchases users make in my mobile app.

I'm using the Google.Apis.AndroidPublisher.v3 NuGet package with the following code to verify an in-app purchase.

public async Task<bool> IsValidPurchase(string bundleId, string receiptId, string purchaseToken, string developerPayload)
{
     try
     {
        var isValidPurchase = true;

        var request = _googleService.Purchases.Products.Get(bundleId, receiptId, purchaseToken);
        var purchaseState = await request.ExecuteAsync();

        if (purchaseState.DeveloperPayload != developerPayload)
            isValidPurchase = false;

        if (purchaseState.PurchaseState != 0)
            isValidPurchase = false;

        return isValidPurchase;
     }
     catch (Exception e)
     {
        throw new Exception(e.Message);
     }
}

I keep getting "Bad Request - Invalid Value" error.

My mobile users will be purchasing auto-renewing subscriptions and after a successful purchase on an Android phone, I get the following response -- see image below:

The verification method asks for bundleId, receiptId, purchaseToken and developerPayload.

The bundleId and purchaseToken are pretty clear so I'm thinking the error I get is either due to receiptId or developerPayload.

I tried using both Id and ProductId values from the response -- see image above -- but I keep getting the same error:

Bad request - Invalid Value

Any idea what's wrong here? Am I using the wrong values? I'd appreciate some pointers here.